The invention relates to shock and vibration protection for tools containing explosive components, such as shaped charges in perforating guns.
One operation that is performed in completing a well is the creation of perforations in a formation. This is typically done by lowering a perforating gun string to a desired depth in a wellbore and activating the gun string to fire shaped charges. The shaped charges when fired create perforating jets that form holes in any surrounding casing as well as extend perforations into the surrounding formation.
Various types of perforating guns exist. One type of perforating gun includes capsule shaped charges that are mounted on a strip in various patterns. The capsule shaped charges are protected by individual containers or capsules from the harsh wellbore environment. Another type of perforating gun includes non-capsule shaped charges, which are loaded into a sealed carrier for protection. Such perforating guns are sometimes also referred to as hollow carrier guns. The non-capsule shaped charges of such hollow carrier guns may be mounted in a loading tube that is contained inside the carrier, with each shaped charge connected to a detonating cord. When activated, a detonation wave is initiated in the detonating cord to fire the shaped charges. In a hollow-carrier gun, charges shoot through the carrier into the surrounding casing formation.
After a perforating gun is assembled, it is transported to the well site, which may be at some remote location. During handling, the perforating gun may be subjected to shock, such as due to accidental drops. Also, as the perforating gun is being transported, such as in a truck or a boat, the perforating gun may continue to be subjected to shock and vibration. After the perforating gun reaches the well site, it is subjected to further handling to prepare it for lowering into the wellbore. Once it is inserted into the wellbore, the perforating gun is typically run thousands of feet into the wellbore. During run in, the perforating gun may collide with other downhole equipment, such as production tubing or casing, or with the wall of the wellbore, which subjects the gun to further shock.
The various shock and vibration loads that are applied to the perforating guns may damage the components inside the guns, including the shaped charges and detonating cords. Referring to FIG. 1, a shaped charge 210 typically includes a main explosive charge 216 and a metallic liner 220, both contained in an outer case 212. A primer charge 214 coupled to the back of the main explosive is typically connected to a detonating cord 224. A detonation wave traveling down the detonating cord 224 transfers energy to the primer charge 214, which in turn initiates the main explosive 216. Detonation of the main explosive 216 causes the liner to collapse to form the perforating jet.
If the shaped charge 210 is subjected to relatively high levels of shock and vibration loads, the liner 220 may crack or fall out of the case 212. Further, the shaped charge case 212 may become deformed by the shock and vibration loads. The detonating cord 224 may also be flattened or severed. Such damage to the shaped charges or detonating cord may cause a perforating gun to fail. When a perforating gun is lowered to a desired depth but for some reason cannot be activated, a mis-run has occurred. This requires that the perforating gun string to be pulled out of the wellbore and replaced with a new gun string, which is time consuming and expensive. Also, retrieving a mis-fired gun from a well is a hazardous operation.
Thus, a need exists for a method and apparatus to protect perforating guns and other types of downhole tools from shock and vibration.
In general, in one embodiment, an apparatus for use in a wellbore includes an explosive device, a housing for the explosive device, and at least one protection barrier for the housing to reduce transmission of an external force load to the housing.
Other embodiments and features will become apparent from the following description, from the drawings, and from the claims.